Vibration may be regarded as any motion that repeats itself after an interval of time, or one may define vibrations as oscillations of a system about a position of equilibrium (Kelly 2007). Examples of vibratory motion include the swinging of a pendulum, the motion of a plucked guitar string, tidal motion, the chirping of a male cicada by rubbing its wings, the flapping of airplane wings in turbulence, the soothing motion of a massage chair, or the swaying of a slender tall building due to wind or an earthquake.